1. Field of the Invention
The present invention relates to a rotor of a helicopter and, more particularly, to a rotor blade of a helicopter for reducing its noises.
2. Description of the Related Art
Especially when the helicopter is to land, the blade of the rotor interacts with the tip vortex, as generated from the tip of the advancing rotor blade, to generate the BVI noises. It is known that the magnitude of the BVI noises depends upon such a planar shape of the tip of the rotor blade as generates the tip vortex.
A rotor 100 of a helicopter is provided, as shown in FIG. 12, with a plurality of rotor blades 101, each of which has a generally rectangular shape, as its tip is shown in FIG. 13, for example, and which rotate on the center of rotation 102.
Especially when the helicopter is to land, the tip trailing edge vortex 103, as generated at the tip of an advancing rotor blade 101a, does not quickly fall so that a retreating rotor blade 101b interacts with the tip trailing edge vortex 103 generated at the tip of the rotor blade 101a, thereby to cause the BVI noises.
As shown in FIG. 14, on the other hand, there is a method for reducing the shock noises by giving a sweepback angle to the tip of the rotor blade 101 to lower the airspeed and weaken the shock waves. However, this tip shape cannot lower the BVI noises.
Thus, many investigations have been made on the tip shape for reducing the BVI noises by changing the tip vortex generations which determine the magnitude of the BVI noises of the helicopter.
As shown in FIG. 15, for example, the tip vortex to be generated is halved and weakened by mounting a small wing 105 having a rectangular shape to the leading edge of the rectangular tip of the rotor blade 101, so that the BVI noises can be lowered.
On the other hand, a rotor blade 110, as disclosed in Unexamined Published Japanese Patent Application No. 4-262994, is provided with a leading blade 115 merging into a base wing 113 of an airfoil section having a leading edge 112, as shown in a top plan shape of a tip portion 111 in FIG. 16.
As shown in FIG. 17, the vortex emanating from the rotor blade 110 is halved into tip vortices of substantially equal intensities, i.e., a vortex 115a emanating from the leading blade 115 and as vortex 116a emanating from a trailing wing 116, as located on the inner side of the leading wing 115, thereby to reduce the BVI noises.
The aforementioned rotor blade 110 of Unexamined Published Japanese Patent Application No. 4-262994 suppresses the influences of the BVI by making a division into the two vortices of substantially equal intensities, that is, the vortex 115a emanating from the leading wing 115 and the vortex 116a emanating from the trailing wing 116.
This construction is characterized in that the span of the leading wind 115 is made larger by 50% or more than the chord length of the base wing 113 by generating the two vortexes of substantially equal intensities to hold these vortexes in the separate state as long as possible. The effect of the interactions of the vortexes is weak, and the leading wing 115 is made slender thereby to make it necessary to enhance especially the strength of the root portion of the leading wing 115.
Therefore, the invention has been conceived in view of those points and has an object to provide a rotor blade of a helicopter, which is intended to reduce the noises by dividing the tip vortex, as might otherwise cause the BVI noises of the helicopter, and by causing the divided tip vortexes to interact positively with each other thereby to diffuse them.
In order to achieve the above-specified object, according to an embodiment of the invention, there is provided a rotor blade of a helicopter, which is attached at its root portion to the rotor head of a rotational drive unit, characterized by comprising: a base wing attached at its root portion to the rotor head of the rotational drive unit; and a plurality of small wings having substantially equal spans and disposed at the tip of said base wing.
According to the invention the vortex generated at the wing tip is divided into the plurality of relatively weak tip vortices generated at the tips of the individual small wings, and the individual small wings have the equal spans. Therefore, the vortices generated at the tips of the individual small wings are close to each other and interfere positively with each other and are weakened and diffused. As a result, when the helicopter is to land, the pressure fluctuations, as might otherwise be caused by the interactions between the retreating rotor blade and the tip vortices generated at the tip of the advancing rotor blade, are drastically reduced to suppress the occurrence of the BVI noises.
According to another embodiment of the invention a rotor blade of a helicopter is characterized in that said small wings include: a front wing having a leading edge merging into the leading edge of said base wing; and a rear wing spaced from said front wing and having a trailing edge merging into the trailing edge of said base wing.
The invention restricts the small wings, specifically the front wing and the rear wing. The vortex generated at the wing tip is divided into the two vortices, i.e., the front wing vortex generated at the tip of the front wing and the rear wing vortex generated at the tip of the rear wing. The front wing vortex generated at the front wing moves over the rear wing backward of the rear wing, and the rear wing vortex generated at the tip of the rear wing also moves backward so that the front wing vortex and the rear wing vortex interact positively with each other and are weakened and diffused to suppress the occurrence of the BVI noises.
In a further embodiment of the invention, said base wing has a rectangular tip shape; in that said front wing has a rectangular wing shape having a chord length of about 20% to 30% of the chord length of said base wing; in that said rear wing has a rectangular wing shape having a chord length of about 30% to 50% of the chord length of said base wing; and in that said front wing and said rear wing have substantially equal spans of about 20 to 30% of the chord length of said base wing and are spaced by about 20% to 50% of the chord length of said base wing.
In yet another embodiment of the invention, the front wing and the rear wing having the rectangular shape are arranged at the base wing having the rectangular tip shape, so that the front wing vortex and the rear wing vortex are efficiently caused to interact with each other and are diffused by setting the chord lengths and the spans of the front wing and the rear wing and the spacing between the front wing and the rear wing to the above-specified ranges.
In still another embodiment of the invention, said base wing has a tip shape of a sweepback angle of about 20 degrees to 40 degrees; in that said front wing has a wing shape having a chord length of about 20% to 30% of the chord length of said base wing and a sweepback angle of 20 degrees to 40 degrees; in that said rear wing has a wing shape having a chord length of about 30% to 50% of the chord length of said base wing and a sweepback angle of 20 degrees to 40 degrees; and in that said front wing and said rear wing have substantially equal spans of about 20% to 30% of the chord length of said base wing and are spaced by about 20% to 50% of the chord length of said base wing.
According to the invention, the base wing has the sweepback angle, and the front wing and the rear wing have the sweepback angle, hence, the airspeed of the wing tip is decreased to damp the compressibility of the air thereby to weaken the shock waves and to improve the transonic characteristics.
In yet another embodiment of the invention, said base wing has a tip shape of a sweepback angle of about 20 degrees to 40 degrees; in that said front wing has a tapered wing shape having a root chord length of about 20% to 30% of the chord length of said base wing, a sweepback angle of 20 degrees to 40 degrees and a taper ratio of 0.8 or less; in that said rear wing has a tapered wing shape having a root chord length of about 30% to 50% of the chord length of said base wing, a sweepback angle of 20 degrees to 40 degrees and a taper ratio of 0.8 or less; and in that said front wing and said rear wing have substantially equal spans of about 20% to 30% of the chord length of said base wing.
According to another embodiment of the invention, the front wing and the rear wing are given the tapered shape so that the lift distribution of the wing edges is reduced to improve the damping of the tip vortexes better.
In yet another embodiment of the invention, said front wing and said rear wing have an anhedral angle of about 10 degrees to 30 degrees.
According to the invention, the front wing and the rear wing have the anhedral angle. Therefore, the tip vortex, as generated at the tip of the rotor blade while the helicopter is hovering and when the helicopter is to land, is positively released downward so that the interactions between the tip vortex generated by the advancing rotor blade and the retreating rotor blade are avoided to reduce the torque necessary for the rotation.
In another embodiment of the invention, said base wing has a rectangular tip shape; in that said small wings include: a front wing having a leading edge merging into the leading edge of said base wing; a rear wing spaced from said front wing and having a trailing edge merging into the trailing edge of said base wing; and an intermediate wing arranged between said front wing and said rear wing; in that said front wing, said rear wing and said intermediate wing individually have a rectangular wing shape having a chord length of about 25% of the chord length of the base wing; and in that said front wing and said intermediate wing, and said intermediate wing and said rear wing are individually spaced by about 12% of the chord length of said base wing.
According to another embodiment of the invention, the vortex generated at the wing tip is divided into the three weak vortices, as generated at the individual tips of the front wing, the intermediate wing and the rear wing, and these weak vortices are caused to interact with one another and are diffused so that the interactions between the retreating rotor blade of the rotor and the tip vortex generated from the tip of the advancing rotor blade are avoided or drastically reduced to suppress the occurrence of the BVI noises.
In yet another embodiment of the invention, there is included a variable control unit for controlling the incidence angles of said individual small wings variably at every azimuth angles of said rotor.
According to another embodiment the invention, at every azimuth angles varying with the rotation of the rotor blade, the incidence angles of the small wings can be variably controlled to cause the vortices, as generated from the tips of the individual small wings, to interact with each other and can be diffused to avoid the influences of the stall and the drag divergence.